If you've flushed five times and the pH isn't changing, you've got something in the soil that is buffering the pH. I'd consider transplanting into different soil.
Ya GG is right. Ph looks alittle off. Could be a nitrogen lock out too. Looks like the nitrogen is shifting between the leaves. Would run more ph test and soil test to figure this out.
I have the same problem but it seems only to happen to the leaves close to bottom. The top leaves unless it is the top most set of leaves are starting to get black spots and they wither and crumble when touched.